Comment by relistan
As a ham radio operator who still likes paper QSL cards, I’ve bought international reply coupons from Swiss Post because they still sell them and you can order them online (I live in Ireland but here they no longer sell them). Any postal service that is a member of the Universal Postal Union (nearly all of them are AFAICT) is obliged to accept them. They work in most places.
Hey, I'm KG8ORS. It's been aong time since I've sent a QSL card (and I've never received one), but it's where I first heard about IRCs.
I agree that UPU members are required to accept them. PostNL closed all of their post offices a few years ago in favor of contracting out to businesses, who may not be familiar with them.
It looks like Omniva (aka/fka Eesti Post) also ships worldwide and charges in Euro.